The following meanings for the abbreviations used in this specification apply:
EBB—Eigenvalue Based Beamforming
eNB—evolved node B
EVD—Eigen Value Decomposition
PAPR—Peak-to-Average Power Ratio
POS—Power Overshoot
LWF—Limited Waterfilling
BS—Base Station
Embodiments of the present invention relate to beamforming. Beamforming is achieved by weighting the transmit signal for multi-antenna based on estimated channel information to make the signal combined coherently in the air and increase the power gain in the direction of the desired user. Eigenvalue Based Beamforming (EBB) is one of the beamforming methods using Eigen Value Decomposition (EVD) to compute the weights.
FIG. 4 shows an example of an antenna array with 4 X-pol elements. For this antenna array, the 1×4 dominant eigen vector resulted from EVD is used as the weight vector and applied to both polarization. The phase information of which are to compensate the phase differences of the channel responses for different X-pol elements while the amplitude information are to optimally distribute the total transmit power among the X-pol elements according to the channel estimates. As for both polarizations the same 1×4 weight vector is used, in the following only one of the polarizations is shown for illustration.
A problem regarding this way of determining the transmit power for each antenna element of the antenna array is rooted from the conflict between EBB's power distribution nature and hardware characteristics limitation. EBB distributes the total transmit power according to the channel estimates at different antenna elements, say more power is allocated to the antenna element with better channel response, and generally the channel responses are not equally good across the antenna elements. On the other hand, we have a uniform antenna array that is each antenna element with the same power radiation capability. Therefore, when we apply the beamforming weights to the transmit antenna array, some of the antenna elements work out of the linear amplifier range, this brings high PAPR problem and may trigger an alarm or lead to broken antenna if, sometimes, one of the antenna elements gets a very good channel response and most of the power is allocated to it, which is the so-called Power Overshoot (POS) issue.
This is illustrated in FIG. 5, in which an example is shown in which more transmit power than the per-antenna (element) transmit power capability is allocated to antenna element 3.
According to the prior art, to avoid the POS issue, generally the total transmit power of beamforming is decreased with a fixed scaling. This measure is introduced not only for POS, but also to mitigate the severe inter-cell interference when beamforming is switched on. In addition, if the problem is still there with the decreased total power, a fixed safe weight vector will replace the one computed from EBB. Consequently, both the phase and amplitude weighting information are lost. Another way to tackle the problem is that the overshot power is simply removed to match the antenna capability, the so called power capping, which makes the total transmit power unstable and vary with respect the channel condition. There is also a method removing the amplitude information of the weight vector and let all antenna elements transmit with maximum power, namely phase only method. This method is not a good option from the cost-efficient point of view; also it brings severe inter-cell interference.
Hence, there is need for an improved approach to avoid the power overshot issue.
FIG. 6 shows a diagram in which the above prior art methods to avoid the POS issue are illustrated. The last row shows the original weight vector, the penultimate row shows the fixed sector beam method, the second row shows the phase only method, and the first row shows the capping method.